Question: $ A = \left[\begin{array}{rr}2 & 2 \\ -1 & -1\end{array}\right]$ $ D = \left[\begin{array}{rrr}1 & -2 & -1 \\ 4 & 3 & -1\end{array}\right]$ What is $ A D$ ?
Answer: Because $ A$ has dimensions $(2\times2)$ and $ D$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ A D = \left[\begin{array}{rr}{2} & {2} \\ {-1} & {-1}\end{array}\right] \left[\begin{array}{rrr}{1} & \color{#DF0030}{-2} & \color{#9D38BD}{-1} \\ {4} & \color{#DF0030}{3} & \color{#9D38BD}{-1}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ A$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ A$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ A$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{2}\cdot{1}+{2}\cdot{4} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ A$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{2}\cdot{1}+{2}\cdot{4} & ? & ? \\ {-1}\cdot{1}+{-1}\cdot{4} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ A$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{2}\cdot{1}+{2}\cdot{4} & {2}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{3} & ? \\ {-1}\cdot{1}+{-1}\cdot{4} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{2}\cdot{1}+{2}\cdot{4} & {2}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{3} & {2}\cdot\color{#9D38BD}{-1}+{2}\cdot\color{#9D38BD}{-1} \\ {-1}\cdot{1}+{-1}\cdot{4} & {-1}\cdot\color{#DF0030}{-2}+{-1}\cdot\color{#DF0030}{3} & {-1}\cdot\color{#9D38BD}{-1}+{-1}\cdot\color{#9D38BD}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}10 & 2 & -4 \\ -5 & -1 & 2\end{array}\right] $